Internal combustion engines, such as those utilized in motor vehicles, include multiple sensors designed to monitor engine components and allow an engine controller to optimize engine operations for minimum fuel consumption. The sensors transmit sensed data to the engine controller at a certain magnitude. If electrical noise from the engine exceeds that magnitude, the sensor signal can be unreadable by the engine controller.
A typical noise signal originating from an engine is composed of many components from different sources within the engine. The engine harmonic noise is predictable with peaks occurring at specific orders and occurs as a result of both combustion noise and mechanical noise. Some engine harmonic noise arises due to mechanical imbalances such as cylinder to cylinder inertia differences. Combustion noise, however, is driven by imbalances in fueling, intake air, and spark timing factors.
The amplitude of one factor of the engine harmonic noise is proportional to the engine speed, with an increased engine speed directly resulting in an increased amplitude of the harmonic noise. The frequency band of this harmonic noise is at the same harmonic frequency as a paired cylinder misfire pattern. Paired cylinders are the cylinders fired 360 degrees apart (for an even firing engine). The paired cylinder misfire pattern is a cylinder misfire sensor signal used to detect paired engine cylinder misfires. It is also possible for combustion noise patterns that are not completely proportional to speed to overlap with the paired misfire pattern. This relationship between engine speed and the amplitude of engine harmonic noise decreases the signal-noise capability of paired cylinder misfire detection when the engine is operating at high engine speed. In some engine designs, the engine load has a similar affect on the electrical noise to the effect of engine speed.
Some engine controllers utilize crankshaft based misfire detection. Crankshaft based misfire detection employs a digital filter to remove noise from the misfire detection signal and enable misfire signal processing. Since the engine harmonic noise can be at the same frequency as the misfire detection signal, it is impossible for a simple digital filter to remove the engine harmonic noise without removing the paired cylinder misfire signal under standard crankshaft based misfire detection. This is due to the fact that both noise and misfire signal fall within the pass band of the digital filter.